1. Field of the Invention
The present invention relates to a lithium-ion-conductive substance which can be used as an electrolyte of solid electrochemical devices, such as solid-electrolyte batteries, capacitors and solid electro-chromic display units. Moreover, the present invention relates to a solid-electrolyte lithium battery as well.
2. Description of the Related Art
Solid-electrolyte lithium batteries of high safety have been developed as the new-generation lithium secondary battery. The materials for making the solid electrolytes are required to exhibit conductivity as high as possible, and to be stable chemically as well as electrochemically. Crystalline solid electrolytes are most desired materials in terms of stability. However, no crystalline substances of high conductivity have yet been discovered.
The studies on solid electrolytes are the history of investigative researches on how to stabilize the high ionic conduction, which is usually present at elevated temperatures, at low temperatures; and how to synthesize materials exhibiting the high ionic conduction at low temperatures. The researches started with crystalline substances, and have developed into glassy and polymeric substances.
In the development of the next-generation high-energy-density lithium battery, the following are reported: an inorganic glassy electrolyte of high ionic conductivity and of good chemical-and-electrochemical stability; an Li.sub.2 S-SiS.sub.2 -Li.sub.3 PO.sub.4 solid electrolyte disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 4-202,024; and an Li.sub.2 S-GeS.sub.2 -LiI glassy solid electrolyte disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 59-151,770.
The advantages of the glassy solid electrolyte are as follows: it is easy to synthesize the glassy solid electrolyte; it is relatively easy to carry out the material designing of the glassy solid electrolyte; the glassy solid electrolyte exhibits the isotropic ionic conduction; and the glassy solid electrolyte can be readily turned into an actual electrolyte. In particular, when the cooling speed of the glassy solid electrolyte is controlled, the glassy solid electrolyte exhibits an expanded vitrification range so that it is possible to attain compositions and structures which are appropriate for the ionic conduction of lithium ion.
It has been said that the crystalline solid electrolytes and the amorphous solid electrolytes depend on different basic ideas in terms of the material designing. The amorphous solid electrolytes exhibit the degree of freedom in the material designing far greater than the crystalline solid electrolytes. However, if the crystalline solid electrolytes can form structures which are adequate for the ionic conduction, there can exist substances which are far superior to the amorphous solid polymer electrolytes in terms of the ionic conductivity and the chemical-and-electrochemical stability.
Crystalline Rb.sub.4 Cu.sub.14 I.sub.7 Cl.sub.13 is a substance which has exhibited the maximum ion conductivity at room temperature so far. The ion conductivity of crystalline Rb.sub.4 Cu.sub.14 I.sub.7 Cl.sub.13 is better than those of the corresponding glassy silver solid electrolytes and copper solid electrolytes by a couple of digits, and additionally is larger than those of liquid electrolytes at room temperature. Considering the relationship between the crystalline solid electrolytes and amorphous solid electrolytes, it is possible to think of the presence of corresponding crystalline solid electrolyte, since there exist the glassy solid electrolytes whose structures are based on SiS.sub.4 and PO.sub.4 tetrahedrons. Moreover, on the analogy of the silver or copper ion conductors, it is natural that such crystalline solid electrolyte would exhibit a much higher ion conductivity than the glassy substances.
A substance which is famous as a ceramics ion conductor is an oxide, which is usually referred to as "LISICON". In the substance, a framework structure is formed. The framework structure includes SiO.sub.4, PO.sub.4 and ZnO.sub.4 tetrahedrons. Lithium ions diffuse in the framework structure. These oxide substances are well known, since they exist as minerals on the earth. However, crystalline substances whose structures are based on SiS.sub.4 and PS.sub.4 tetrahedrons have not been known yet to exist as such oxide substances, since they are not present in nature. Hence, the inventor of the present invention has decided to study these novel substances.
Thus, the present inventor has started investigating the crystalline substances whose structures are based on SiS.sub.4, PO.sub.4, PS.sub.4 and PN.sub.4 tetrahedrons. Then, he aimed at constructing substance systems which are appropriate for the lithium ion diffusion, and at applying them to solid electrolytes for secondary batteries.